Waste sulphuric acid streams are generated in many industrial operations including the leaching of ores, such as titanium dioxide, the scrubbing of furnace gases in smelters, tail gas scrubbing in power plants, the pickling of metals such as steel and many other similar operations. Until recently, many of these streams were discharged to the environment such as by discharging them to oceans or rivers, or the streams were neutralized by the addition lime to form large piles of heavily contaminated gypsum. These waste streams typically comprise large volumes of material. For example, in the case of a sulphate process titanium pigment operation, from about 200 to about 300 tons per day of waste acid may have to be processed. In the case of a drying or pickling operation, from about 5 to about 10 tons per day of waste acid may have to be processed. The discharge of such large amounts of waste acid to the environment is no longer acceptable or, in some cases, permitted. Further, due to landfill costs, disposal of the neutralized effluent is undesirable.
One alternative approach comprises subjecting the waste acid stream to a pre-concentration step and subsequently, after filtration, passing the stream through a crystallization unit to produce a processed waste acid stream containing iron sulphate heptahydrate. After the iron sulphate heptahydrate is removed, the processed waste acid stream is then concentrated.
One disadvantage with this process is that the iron sulphate is removed as iron sulphate heptahydrate. Iron sulphate heptahydrate is a gelatinous precipitate which tends to plug or foul process equipment. Accordingly, the production of this precipitate requires special handling techniques and more frequent, time consuming cleaning of the process equipment.
A further disadvantage of this process is that all metal sulphates do not crystallize out at the same rate. Some will concentrate in the liquor phase. Accordingly, additional process steps are required if the waste acid is to be recycled.
An alternate process comprises passing the waste acid streams to a pre-concentration step and then subjecting the concentrated waste acid to a spray drying process. The spray drying process results in the production of vaporized sulphuric acid and water with the dissolved salts being removed as a dry powder. The vaporized water and sulphuric acid are subjected to a partial condensation step to produce conventional strength acid. A disadvantage with this process is that the dry solids are very hygroscopic and corrosive when wet. A further disadvantage is that spray drying equipment has a high capital cost. In addition, due to the substantial energy input which is required to vaporize the entire waste acid stream, the operating costs of the spray drying operation are relatively high.